1. Field of the Invention
The present invention relates to a clutch system for power transmission in a motor driven telescopic antenna installed in automobiles, etc.
2. Prior Art
In general, this type of motor driven telescopic antenna is constructed as shown in FIG. 5. That is, it basically comprises an electric motor unit 1, a power transmission unit 2 and a telescopic antenna unit 3. For the electric motor unit 1, usually a torque motor, etc. is used. The power transmission unit 2 includes a power converter for converting the rotary motion of the electric motor unit 1 to linear motion, and a clutch system for releasing the power transmitting operation during overload of the telescopic antenna unit 3. In the telescopic antenna unit 3 a drive cord 3a is provided, and by the motion of reciprocal feeding of this drive cord 3a, that is performed by the linear motion of the power transmission unit 2, the antenna is extended or retracted.
As the clutch system mentioned above, various types of them have been used, and a representative one is that having the structure as mentioned below. That is, a main driving clutch disc, that rotates by receiving the driving force from the electric motor, and a follower clutch disc that is coupled under load with the former are positioned to be pressed into engagement. During the telescopic operation of the antenna, the antenna is extended or retracted by the driving force applied to the antenna side by means of the friction transmitting force between the foregoing main driving clutch disc and the follower clutch disc. At the time when the telescopic motion of the antenna is completed and the follower clutch disc stops, the main driving clutch disc slips. Thereafter, by actuating a limit switch, etc., the rotation of the main driving clutch is stopped.
In the clutch system having the structure as mentioned above, the transmission of the driving force from the electric motor side to the antenna side depends entirely on the transmission of the friction between the main driving clutch disc and the follower clutch disc. Also, the setting of the clutch pressure (corresponding to the force in the reciprocal feeding motion of the drive cord 3a) is done depending on the setting of the pressurizing force of the above. Accordingly, when dirt, contaminants, etc. become adhered to the clutch portion, that is, between the main driving clutch disc and the follower clutch disc, the clutch is not released even though the clutch pressure is above the set value (overloaded), thereby imposing an excessive load on the electric motor, and in the worst case, burning of the electric motor has been caused.